Radio frequency output circuits and output windows



3,1 13,238 RADIO FREQUENCY OUTPUT CIRCUITS AND OUTPUT wmnows Dec. 3,- 1963 c. s. BIECHLER ETAL Filed May 29, 1961 FIG.2

INVENTORS R EM LS HR Y C M E EEMY N m us .S SDDT T S R EAA mama ARAR H0 CC Y B United States Patent Ofitice 3,l H238 Patented Bee. 3, llfi3 3,113,233 RADIG FREQUENCY GUTFUT ClRtIUlTS AND UUTPUT WKNDOWS Charles S. Biechler and Cortland S. Pearsfll, lies Altos, Armand Staprans, Mountain View, and Robert S. Symons, Mcnlo Park, Calif., assignors to Varian Associates, Palo Alto, Calif., a corporation of California Filed May 29, 1961, Ser. No. 113,235 14 Claims. (Cl. 315-39) This invention relates in general to radio frequency output circuits and windows and, more particularly, to a novel wide band output circuit and output window for radio frequency generators including microwave generators and lrlystrons.

The life of a high power tube, heretofore, has been seriously limited by the unreliability of output windows. Failure of the output window normally causes a leak, allowing the vacuum within the tube to go up to atmospheric pressure; thereby rendering the tube inoperative and permanently damaging the cathode, necessitating its replacement.

Another problem associated with radio frequency generators is that of providing a constant impedance output circuit which will fit together with mechanical ease to the output cavity, the focusing magnet and the collector in, for example, a klystron, without interfering with those components.

It is, therefore, the object of the present invention to provide a novel radio frequency output circuit and window for use with radio frequency generators, such as klystrons, which will provide a constant impedance to the beam of the radio frequency generator and further provide a wide band waveguide output window suitable for high peak and average power transmission.

One feature of the present invention is the provision of a wide bandwidth, mode free output window which has a smaller surface area dimension than the crosssectional area of the waveguide.

Another feature of the present invention is the provision of a substantially constant impedance output circuit having a nearly constant response over a wide bandwidth.

Another feature of the present invention is the provision of novel support means for supporting the output window in such a manner as to reduce stress in the vacuum seal and to provide uniform stress distribution around the window.

Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawing wherein,

FIG. 1 is an isometric view, partly cut away, of a novel output waveguide circuit and its associated output window,

FIG. 2 is a cross sectional view of the output window taken at line 22 in FIG. 1,

FIG. 3 is an enlarged fragmentary view of the portion of FIG. 1 inscribed by the circle 3-3, and

FIG. 4 shows a diagram in schematic form of the output circuit including the output window of the present invention.

Referring now to FIGS. 1, 2, 3 and 4 of the drawings, there are shown the essential details of the present invention including the novel output window as seen in FIG. 2. Cut-away sections have been made in FIG. 1 of the output drift tube, the coupled cavit the window matching iris and the output window itself. The output circuit of the present invention employs a broadband multiple tuned circuit as taught in a copending application of Robert L. Jepsen et al. entitled High Frequency Tube Apparatus and Coupled Cavity Output Circuit Therefor, now US. Patent 3,028,519, filed January 2,

1959, issued April 3, 1962 and assigned to the same assignee.

An output cavity 11 of, for example, a high power klystron, such as an amplifier or oscillator, is coupled to an external load (not shown) through a semi-circular output iris opening 12 cut through a pole piece iii, and through an output waveguide 13. Output cavity 11 is centrally apertured for passage of an electron beam which passes through output drift tube 1.5 and into an electron collecting structure 14-. A slanted iris wall 16, a conical collector shield or conical wall member 2% and portions of the walls of the waveguide 13 form a coupled cavity 3d. The slanted iris wall 16 and a metal rounded post 1'7 secured to wall 16 and extending inwardly therefrom, controls the resonant frequency and Q (Q external Q) of the coupled cavity 36 such that an equal ripple resistance response is seen at the interaction gap of the output cavity resonator 11 about the optimum resistance over the frequency band of interest. By having the coupled cavity 30 of the output waveguide 33 wrapped around conical collector shield or conical wall member 2% broadbanding is accomplished without a long waveguide run or breaking of the magnetic structure which surrounds the cavities (not shown) of the radio frequency generator, for example, a idystron, generally shown at 35. The cone angle of conical collector shield. 29 is designed in accordance with the universal beam spread curve so that a larger collector could be utilized. if so desired. This would allow a higher power density electron beam to be intercepted by a larger collector. The bend. in waveguide 13 is substantially rellectionless; however, it is noted that the position of iris wall 16 is affected by the bend due to interaction between the electrical wave passing through the bend portion of waveguide 13 and iris wall 16. It is also noted that rounded post 17 acts much like a tuning stub in that it has a capacitive effect on the resonant frequency of coupled cavity 39. The rounded post 17 further serves the function of providing a better match at iris opening 12. A second iris 18 is provided in waveguide 13 to aid in the adjustment of the Q of the coupled cavity 3i). Channeled members 32 are positioned around the waveguide 13 to provide greator rigidity and strength.

The novel output window assembly, shown generally at 21, comprises a flat, oval shaped ceramic window or energy permeable member 22 which has been metallized and sealed, as by brazing, to the base of an annular, flanged window support cup 23, as of, for example, Kovar. The height of window 2 2 is somewhat larger than the height of waveguide 13 to provide necessary seal detail which is re-entrant into the waveguide wall. The flanged lip portion of the window support cup is received in a vacuum tight manner, as by heliarc-welding, to the flange portion of a flange support cylinder 23. The base portion of flange support cylinder 23' is brazed to one Wall of a channel 24 provided, as by machining, in a window support member or flange 31. A cover plate flange 29 is secured to the outer edge of flange member 31, by, for example, bolts, for connecting Waveguide 13 to an external load. To increase the strength of the ceramic-to-i lovar seal between window 22. and cup 23, best seen in FIG. 3, a ceramic back-up ring member 25 is brazed to the bottom side of support cup 23 thereby sandwiching the support cup between ceramic ring 25 and ceramic window 22 to reduce shearing stresses on the braze joints due to the differential expansion between the Kovar and ceramic. A support ring 26 of soft metal, for example, copper, is secured as by brazing to window support member 3 within the channel 24. The harder ceramic member 25 imbeds itself into metal ring support 26 to provide a uniform stress distribution around the window. An RF. shield 27 formed by the other wall of the channel is provided to prevent an RF. electric field concentration at the inner rim of the base of support cup 23 and to greatly reduce RF. fields in the re-entrant seal region formed by the channel 24 in window support flange 31.

it is noted from FIG. 2 that the width of the window 22'. is narrower than the width of the waveguide 13 cross section. This narrow and relatively thin window prevents higher order mode resonance found in windows which are larger in surface area than the waveguide cros-section area. The dimensions of that portion of the window through which the passes is determined-by the height of the waveguide and the width of the window so that the window is resonant at the desired output frequency. This provides a low Q resonant window (somewhat less than 10), without the presence of higher mode resonances. The bandwidth is further improved by the small inductive matching iris 18 which is placed approximately /sa from the window itself (k wavelength in the waveguide), thereby presenting a flat transmission line over the operating band of frequencies. The total response of the window and iris combination has a VSWR (voltage standing wave ratio) of less than 1.1 over a 17% bandwidth. This window is particularly suited for high average and peak power densities because the surface area of the window is nearly as large as the cross section of the waveguide, and also because the electric field is nearly parallel to the window, thus discouraging multipactor, arcing, or other types of electronic discharges. A window which is 16 inches wide by 12 inches high and /2 inch thick has been tested successfully to 16 megawatts of peak power and 30 kilowatts of average power in the frequency range of 400 to 450 megacycles. The thickness of the window is governed by mechanical strength considerations as it must withstand gas pressure when one side of the waveguide is at a high vacuum, in the order of 10* mm. of Hg. Also the dielectric constant of the window and frequency of the energy must be considered.

What we have described is a novel output circuit incorporating a large ceramic output window making posisble radio frequency generators, such as klystrons, of higher power than was possible with cylindrical windows.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

l. in a radio frequency electron tube, means for extracting radio frequency energy therefrom, including a waveguide member in which said radio frequency energy may pass, support means including a support member transversely positioned within said waveguide for supporting a radio frequency energy permeable member in a vacuum tight manner, said support means including a flanged annular cup member secured at its flanged lip to said support member and secured at its base to said energy permeable member, a first annular ring member backing up said flanged annular cup member and a second ring supporting said first ring member on one side and secured on the other side thereof to said support member, said first ring and said second ring members reducing stress on said energy permeable member.

2. The combination according to claim 1 further including a radio frequency shield between said support member and said energy permeable member to prevent an e co ic field concentration between said energy permeable member and said annular cup member.

3. The combination of claim 1 wherein said energy permeable member is a ceramic window.

4. The combination according to claim 1 wherein said energy permeable member has a surface area less than the cross-sectional area of said waveguide.

5. The combination according to claim 1 further including an inductive matching iris positioned within said waveguide member approximately A from said energy permeable member, where equals the wavelength of the radio frequency energy within said waveguide.

6. in a radio frequency electron tube, means for extracting radio frequency energy therefrom, including, a wave guide member for passing said radio frequency energy, support means including a support member transversely positioned within said waveguide for supporting a radio frequency energy permeable member in a vacuum tight manner, s id permeable member having a plane area less than the cross-section area of said waveguide, said support means including a flanged annular cup member and a flanged cylindrical member, said cup member secured at its base to said energy permeable member and secured at its flanged lip to the flanged portion of said cylindrical member, said last named member secured at its base portion to said support member, a first annular ring member backing up said flanged annular cup member, a second ring supporting on one side said first ring member and secured on the other side thereof to said support member, said first and second ring members serving to reduce stress on said energy permeable member and a radio frequency shield between said support member and said energy permeable member to prevent an electric field concentration between said energy permeable member and the base of said annular cup member.

7. The apparatus according to claim 6 further including an inductive matching iris positioned within said waveguide member approximately from said energy permeable member, where M. equals the wavelength of the radio frequency energy within said waveguide.

8. The apparatus according to claim 6 wherein said second ring is of soft metal which serves to provide uniform stress distribution for said energy permeable memher.

9. The apparatus according to claim 6 whereby said energy permeable member is a ceramic window.

10. High frequency amplifier tube apparatus including, means for forming and projecting a beam of electrons over an elongated predetermined beam path, a collector structure disposed at the terminal end of the beam path for collecting the beam, a plurality of wave propagating structures disposed along the beam path intermediate its length for electromagnetic interaction with the beam, means for modulating the beam with input signals to be amplified, a first output resonator structure surrounding the beam for extracting amplified signal wave energy from the beam, a second resonator structure coupled to said first output resonator, an output hollow waveguide structure coupled to said first output resonator via the intermediary of said second resonator, a wave permeable gas tight RF. window member disposed outside of said first and second coupled resonators in said output waveguide for vacuum sealing said output hollow waveguide structure, said second resonator being formed by a section of hollow waveguide at least partially wrapped around said collector structure, said second resonator waveguide structure having a pair of opposed and spaced apart broad side walls and a pair of opposed and spaced apart narrow side walls, and one of said broad side walls of said hollow waveguide structure of said second resonator being defined by an iris wall projecting into said output hollow waveguide structure.

11. The apparatus according to claim 10 wherein one of said broad side walls of said second resonator waveguide structure is of frustro-conical shape at least partially surrounding said collector structure.

12. The apparatus according to claim 10 wherein said second resonator structure includes a post projecting into said second resonator for influencing the resonant frequency of said second resonator.

13. The apparatus according to claim 11 including a 5 6 post extending into said second resonator from said iris References Cited in the file of this patent wall.

14. The apparatus according to claim 12 including an UNITED STATES PATENTS inductive iris disposed in said output waveguide struc- 2,815,467 Gardner Dec. 3, 1957 ture between said second resonator and said wave perme- 5 2,843,795 B on d1 W et aL y 1958 able window member for broadbanding said window mem- 2 878 415 Gorm y et a1 Mar 17 1959 her. i 

1. IN A RADIO FREQUENCY ELECTRON TUBE, MEANS FOR EXTRACTING RADIO FREQUENCY ENERGY THEREFROM, INCLUDING A WAVEGUIDE MEMBER IN WHICH SAID RADIO FREQUENCY ENERGY MAY PASS, SUPPORT MEANS INCLUDING A SUPPORT MEMBER TRANSVERSELY POSITIONED WITHIN SAID WAVEGUIDE FOR SUPPORTING A RADIO FREQUENCY ENERGY PERMEABLE MEMBER IN A VACUUM TIGHT MANNER, SAID SUPPORT MEANS INCLUDING A FLANGED ANNULAR CUP MEMBER SECURED AT ITS FLANGED LIP TO SAID SUPPORT MEMBER AND SECURED AT ITS BASE TO SAID ENERGY PERMEABLE MEMBER, A FIRST ANNULAR RING MEMBER BACKING UP SAID FLANGED ANNULAR CUP MEMBER AND A SECOND RING SUPPORTING SAID FIRST RING MEMBER ON ONE SIDE AND SECURED ON THE OTHER SIDE THEREOF TO SAID SUPPORT MEMBER, SAID FIRST RING AND SAID SECOND RING MEMBERS REDUCING STRESS ON SAID ENERGY PERMEABLE MEMBER. 